What should be done to activate the secondary FMGC mode?

Activating the secondary Flight Management and Guidance Computer (FMGC) mode involves ensuring that at least one autopilot is selected. This allows the aircraft to utilize the alternate navigation functionalities available within the FMGC system. The secondary mode serves as a backup to the primary navigation and autopilot systems, enabling the aircraft to maintain control and navigate effectively in the event of a primary system failure.

Selecting one autopilot ensures that the flight management system can still function, albeit in a limited capacity compared to when both autopilots are operational. This is critical for maintaining safe flight operations and allowing for continued navigation and guidance, which are essential during various phases of flight or in non-normal situations.

The other options do not contribute to the activation of the secondary mode in the same way. Turning off both autopilots would disable the flight control systems entirely, while manual input on the MCDU would not suffice to trigger the secondary mode without an active autopilot. Setting navigation to ADF pertains to a different navigation function entirely and is not related to the operation or the mode selection of the FMGC.